The present invention relates to an arrangement and a method for converting thermal energy to mechanical energy.
When fuel is burnt in a combustion engine in a vehicle, chemical energy is converted to mechanical energy to propel the vehicle. However, a substantial part of the chemical energy is converted to thermal energy which is released to the surroundings in various ways. An example of this is the thermal energy in the exhaust gases which are discharged to the surroundings. Another example is the thermal energy present in various kinds of warm media in the vehicle which is actively cooled away in various types of cooling devices. An example of such warm media is the coolant in cooling systems which cool the engine and possibly other components of the vehicle. The warm coolant is usually cooled in a radiator by surrounding air at the front portion of the vehicle. Many combustion engines are supercharged, which entails compressed air being supplied to them. The charge air is cooled in at least one charge air cooler before being led into the engine. To reduce discharges of nitrogen oxides, part of the exhaust gases may be recirculated. The recirculating exhaust gases are cooled in at least one EGR cooler before being mixed with the charge air and led into the engine etc.
US 2003/0145583 refers to an example of a so-called WHR (waste heat recovery) system in a vehicle. WHR systems are used to convert thermal energy to mechanical energy. A WHR system comprises a circuit with a pump which circulates water in a line circuit. The line circuit comprises an evaporator in which the water is vaporised by heat from the engine's exhaust gases, and a turbine which is driven by the vapour. When the vapour expands through the turbine, part of its thermal energy is converted to mechanical energy. A power transmission system comprising inter alia a planetary gear transmits the turbine's motion to an output shaft of the engine and/or to an electrical energy generator.